Apparatus for suction and discharge of material as mentioned above are used, for instance, when making excavations in areas where electricity cables, telecommunication cables and the like are buried in the ground, and where an ordinary excavator may cause damage by breaking such cables by mistake. The material sucked up consists of stones, macadam, gravel, sand, earth etc. Such apparatus can also be used to suck up material in liquid form, such as mud and wet clay. One field of application is to suck up ballast adjacent to the rail when reconditioning railroad lines.
In a prior-art procedure, the ballast is sucked through a hose into a vacuum container which is placed on a rail vehicle. When the container is full, doors in the sides of the container are opened, and the ballast is discharged along the vehicle sides. There is usually some kind of guide plates arranged at the side of the vehicle below the doors in the container sides to direct the discharge in the desired direction somewhat obliquely away from the container. This prior-art apparatus cannot be used in cases where the sucked-up material is to be discharged behind the vehicle, for example to be able to convey it onto a collecting vessel.
U.S. Pat. No. 5,709,270 discloses another apparatus arranged on a rail vehicle for drawing in ballast by suction, for example, when renovating railroad lines. The ballast is drawn up by suction to a vacuum container which, when being full, is opened via a door in the bottom of the container. The ballast is discharged onto a conveyor belt running under the vehicle and discharging the ballast at the rear end of the vehicle, optionally onto another conveyor belt and then onto another container.
A difficulty in this prior-art apparatus is that the conveyor belt for conveying material to the rear end of the vehicle is arranged under the container and the other parts mounted on the vehicle, such as drive means etc, which makes the space for the conveyor belt very limited in the vertical direction. This makes it difficult to reach the conveyor belt for maintenance and repair. Moreover, the belt may easily come to a standstill when material is pinched between the conveyor belt and the lower parts of the superposed devices.
In all these prior-art apparatus, problems arise when discharging wet material since this sticks to the inner walls of the container, thus making it difficult to entirely empty the container. Low temperatures will also cause inconvenience when discharging, for instance, blue clay. The same problem arises when discharging, for instance, macadam according to the prior-art technique, when the material is stopped at the opening of the container and is retained above the opening owing to so-called bridging in the material according to the principle used to build stone bridges in former times.
This problem has been solved by arranging vibrators to vibrate the container so as to make the material come loose from the walls. However, this easily results in all the material falling out of the container in an uncontrolled manner, which is unfavourable.
The fact that the discharging occurs in an uncontrolled manner, both with and without the use of vibrators, means that it is not possible to control the speed of the material flowing out of the container once the door in the bottom of the container or the doors in the sides of the container are open. Nor is it possible to interrupt a discharging process, for instance in the case of a near-accident.
The uncontrolled discharging may also give rise to great strain on the conveyor belt receiving the material flowing out. In many cases a ketchup effect occurs in discharging, which causes a momentarily very high load on the conveyor belt or other devices receiving the discharged material. Problems with build-up of dust may also arise when dry material is discharged too quickly from the container. Dust build-up may cause negative environmental and health effects.
In another known procedure, the container is instead tilted for discharging and the material is discharged behind the vehicle. Also in this case, the problems mentioned above in connection with quick emptying may arise. When tilting the container, problems will also arise when driving through tunnels having a limited height, where discharge can be made difficult or prevented by there not being sufficient space in the vertical direction for tilting of the container.